Dry etching

ABSTRACT

A metallic thin film of copper, silver, gold, or one alloy selected from alloys containing as a main component at least one of these metals is etched by plasma of an etching gas containing at least nitrogen oxide while being reacted with the plasma, whereby making it possible to fine-process electrically conductive materials, heat-transfer materials and electric-contact materials made of copper, silver, gold or an alloy containing as a main component at least one of these metals.

This application is a 371 application of PCT/JP99/06761 filed Dec. 2,1999.

TECHNICAL FIELD

The invention of the present application relates to a dry etching. Morespecifically, the invention of the present application relates to a dryetching method capable of fine processing an electrically conductivematerial, a heat transfer material, an electric contact material, etc.,consisting of copper, silver, gold, or an alloy containing as a maincomponent at least one of these metals.

BACKGROUND ART

In general, full advantage of lithography and etching technologies istaken in the field of electronic devices such as ultra LSIs or magneticdevices, and these devices are fabricated by combining these techniques.

The etching technique is a technique for fabricating a device whichcomprises transferring a resist pattern produced by lithography onto anobject to be processed, i.e., to a semiconductor thin film, a magneticthin film, etc., and includes methods such as wet-chemical etchingmethod, argon ion milling method, and reactive ion etching method. Amongthese etching methods, reactive ion etching method is a kind of dryetching method, and is advantageous in that it enables a most precisetransfer of patterns produced by lithography, and that it is suitablefor fine processing. Moreover, it boasts superior etching rate. In viewof such advantages, numerous large integrated circuits and semiconductormemories are fabricated by the reactive-ion etching method.

The reactive-ion etching method comprises placing the work piece in aplasma of a reactive gas while applying an electric field thereto, andphysically and chemically stripping off successive layers of atoms bythe incident ion beams that are irradiated vertically to the surface ofthe work piece. This method enables anisotropic processing cuttingvertically along the boundary of the mask, and hence, it allows transferof fine and sharp patterns.

In case of reactive-ion etching, firstly, the chemically active speciessuch as the ions or radicals of the reactive gases that are generated inthe plasma are adsorbed onto the surface of the work piece and undergochemical reaction to form a layer of chemical products having a lowbonding energy. Since the surface of the work piece are exposed to theimpact of the positive ions that are accelerated in the plasma by anelectric field and which are vertically incident to the surface, thesurface layers that are loosely bonded are successively stripped off bythe sputtering of ions or by the evaporation into vacuum. In thiscontext, the reactive-ion etching process can be regarded as a processin which a chemical reaction and a physical process proceedsimultaneously, and it is characterized by having a selectivity on aspecific substance and having anisotropy as such to cut vertically intothe surface of the object.

However, despite the superiority of the reactive-ion etching method overother methods, no effective means has been found for etching copper orgold that are widely used in the electronics, or for silver that is usedin abundance as a heat conductive material or an electric contactmaterial. The reason for this is that copper, silver, and gold undergoreaction with various types of etching gases such as CF₄, CCl₄, CCl₂F₂,CClF₃, CBrF₃, Cl₂, C₂F₆, C₃F₈, C₄F₁₀, CHF₃, C₂H₂, SF₆, SiF₄, BCl₃, PCl₃,SiCl₄, HCl, CHClF₂, etc., which are developed for etching semiconductormaterials, and form reaction products with a bonding energy far higherthan semiconducting materials. Thus, the reaction products are less aptto be subjected to a sputtering or an evaporation, and cannot be removedin a plasma.

Under the aforementioned circumstances, wet-chemical etching process orargon ion milling process has been conventionally applied to copper,silver, and gold to fabricate, for instance, a thin film magnetic head,a magnetic sensor, a micro transformer, etc. Furthermore, aluminum hasbeen used for the electrodes and interconnections necessary forsemiconductor devices by taking advantage of the ease in applyingreactive-ion etching process at the expense of a high electricresistance and a high heat emission.

DISCLOSURE OF INVENTION

The invention of the present application has been made in the light ofthe aforementioned circumstances, and an object of the invention is toprovide a dry-etching method capable of fine processing an electricallyconductive material, a heat transfer material, an electric contactmaterial, etc., consisting of copper, silver, gold, or an alloycontaining as a main component at least one of these metals.

According to the invention of the present application, the aboveproblems are solved by providing a dry-etching method comprising etchinga metallic surface of copper, silver, gold, or an alloy containing as amain component at least one of these metals by plasma of an etching gascontaining at least nitrogen oxide while being reacted with the plasma.

Furthermore, the invention of the present application provides, as apreferred embodiment, a dry-etching method in which the etching gas is amixed gas of nitrogen oxide and hydrogen or a hydrogen-containingcompound; in which the hydrogen-containing compound is one type or twoor more types of compounds selected from the group consisting ofammonia, hydrocarbons, halogen-containing hydrocarbons, or hydrogensulfide, and the mask material to be used in covering the metallicsurface on etching is the one selected from the group consisting oftitanium, titanium alloys, aluminum, or aluminum alloys.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1(a) to FIG. 1(f) are each a cross section view showing thedry-etching process stage in practicing the process according to theinvention of the present application.

FIG. 2 is a cross section view of an example of a reactive ion etchingapparatus suitably used in the dry etching according to the invention ofthe present application.

FIG. 3(a) to FIG. 3(c) are each an electron micrograph showing the stateof a copper or a gold thin film after being subjected to the dry etchingaccording to the invention of the present application.

The numerals shown in the figures each represent the following:

-   -   1 Glass substrate or a dielectric substrate    -   2 Metallic thin film    -   3 Resist    -   4 Mask    -   5 Reaction vessel    -   6 Deposition protection plate    -   7 High frequency electrode    -   8 Sample holder    -   9 Zero-potential shield    -   10 Counter electrode    -   11 Etching gas inlet    -   12 Etching gas    -   13 High voltage radio frequency power supply

BEST MODE FOR CARRYING OUT THE INVENTION

In the dry etching according to the invention of the presentapplication, a metallic surface formed of copper, silver, gold, or onealloy containing as a main component at least one of these metals isetched by plasma of an etching gas containing at least nitrogen oxidewhile being reacted with the plasma. As described above, there is noparticular limitation on the etching gas to be used in the process solong as it contains at least nitrogen oxide. The nitrogen oxide asreferred herein includes nitrous oxide (N₂O), nitrogen monoxide (NO),and nitrogen dioxide (NO₂). Furthermore, the etching gases refer notonly to pure gases, but they can be mixed gases containing othercomponents. In case of a mixed gas, preferred as the other components tobe mixed with nitrogen oxide is, for instance, hydrogen (H₂) or acompound containing hydrogen. As a compound containing hydrogen, therecan be mentioned as examples, one or two or more of, ammonia (NH₃), agaseous hydrocarbon such as methane (CH₄), a halogen-containinghydrocarbon (i.e., CX_(n)H_(4-n), wherein X represents one or two ormore types of a halogen element selected from F, Cl, Br, or I, and nrepresents an integer of 1 to 3), or hydrogen sulfide (H₂S). Among them,particularly hydrogen is preferred to other hydrogen-containingcompounds. Hydrogen effectively allows nitrogen oxides such as NO₂ tocontribute to the etching; more specifically, when compared with theother hydrogen-containing compounds under the same etching conditions,hydrogen increases the etching rate, and reduces the amount of hydrogenincorporated as an impurity into the object by decreasing the amount ofhydrogen ions bombarded to the object during the etching process.

By using plasma of the etching gases, copper, silver, gold, or an alloycontaining as a main component at least one of these metals can besubjected to etching, and an anisotropic selective processing whichenables cutting along the boundary of the mask can be performed. Fineand sharp patterns can be thereby transferred. More specifically,because the etching gases contain at least nitrogen oxide, the bondingenergy of the reaction products that are formed through the reaction ofcopper, silver, or gold in the plasma becomes sufficiently lower ascompared with the case using etching gases conventionally used forsemiconductor materials; hence, the reaction products are therebyrendered sensitive to the sputtering and easily removed. In this manner,reactive-ion, etching method is allowed to be applied to copper, silver,gold, or an alloy containing as a main component at least one of thesemetals, and these metals can be efficiently finely positioned at highprecision and at a favorable etching rate.

Thus, the dry etching according to the invention of the presentapplication is found effective in the fabrication of thin film coils forwriting heads of magnetic disks, the fabrication of micro transducers ormicro coils incorporated in magnetic integrated circuits, or in thefabrication of quantum effect devices such as spin scatteringmagnetroresistance device, spin-valve device, ferromagnetic tunnelingdevice, spin field effect device, spin diode, spin transistor, etc., orin the fabrication of thin film coils for micro motors. Furthermore, thedry etching according to the invention of the present application isalso effective in the fabrication of the interconnections among thedevices assembled in three-dimensional large scale silicon integratedcircuits such as CPUs and DRAMs produced from semiconductor silicon.

FIG. 1 is a diagram showing the cross section view of the practicalprocess steps according to the invention of the present application.

<a> First, a metallic thin film (2) to be subjected to the fineprocessing, the metallic thin film being of copper, silver, gold, or analloy consisting as a main component at least one of these metals, isformed on a glass substrate or a proper dielectric substrate (1) bymeans of, for instance, a sputtering method, a vacuum evaporatingmethod, or a plating method, etc. The metallic thin film (2) to beprocessed can be formed at a thickness in a range of several nan meters(nm) to several micrometers (μm) depending on the design of the desiredelectromagnetic device.

<b> Then, a resist (3) is applied on the metallic thin film (2) by meansof coating and the like. A fine pattern is then formed by means ofelectron beams lithography, ion beam lithography, photolithography,etc., followed by development.

<c> A mask (4) is formed thereafter by means of, for example, vacuumevaporation, etc., over the resist pattern thus formed.

<d> The resist (3) is then removed by dissolution by immersing theresulting product in an organic solvent. As a result, a fine mask (4)can be formed on the surface of the metallic thin film (2). The maskthus formed is provided in the shape of the desired device, for example,an electromagnetic device; more specifically, in a coil-like shape or inthe shape of an electric contact pad or of an interconnection to beincorporated in the semiconductor large scale integrated circuit.

Mask materials can be properly selected from those capable of beingsubjected to an etching using a plasma of an etching gas containing atleast nitrogen oxide, which are not consumed and have excellentstability. Among them, preferred are titanium, a titanium alloy,aluminum, or an aluminum alloy.

As titanium or a titanium alloy, exemplified are pure titanium, a Ti-Pdalloy, a Ti-Ta alloy, a Ti-Al alloy, a Ti-Ai-Sn alloy, a Ti-Al-V-Moalloy, a Ti-Al-Sn-Zr-Mo-Si alloy, a Ti-Al-Zr-Mo-Sn alloy, a Ti-Al-Valloy, a Ti-Al-Sn-Zr-Mo alloy, a Ti-Al-V-Sn alloy, a Ti-V-Cr-Al alloy,etc. As aluminum or an aluminum alloy, exemplified are, in addition topure aluminum, an Al-Cr-X alloy (where X represents an additionalelement such as Si, Mn, Mg, etc.), an Al-Mn-Y alloy (where Y representsan additional element such as Mg, Si, etc.), an Al-Mg-Z alloy (where Zrepresents an additional element such as Zn, Si, Cr, Mn, Mg, etc.), anAl-Si-W alloy (where W represents an additional element such as Mg, Cu,Cr, etc.).

<e> Then, by using a reactive-ion etching apparatus, the portion notcovered by a mask (4) of the metallic thin film is removed by usingplasma of the reactive etching gas, and the pattern of the mask (4) istransferred to the metallic thin film (2).

<f> The residual mask (4) is removed thereafter by an ordinaryreactive-ion etching process using reactive gases such as flon (CP₄),carbon tetrachloride (CCl₄), etc.

FIG. 2 is a cross section view of a reactive-ion etching apparatussuitably used in the dry etching according to the invention of thepresent application.

Referring to FIG. 2, a reaction vessel (5) is made of titanium or atitanium alloy, and the inner wall thereof is covered by an depositionprotecting plate (6) made of titanium or a titanium alloy. Various typesof structures that are placed inside the reaction vessel (5) and thatare to be brought into contact with the plasma of the etching are alsomade of titanium or a titanium alloy.

The work place to be processed is attached and fixed on the surface of asample holder (8) fixed to a water-soluble high frequency electrode (7).The surroundings of the high frequency electrode (7) is covered by azero-potential shield (9), such that the high frequency electrode (7)itself may not be subjected to the etching reaction.

A counter electrode (10), which is larger in area as compared with thehigh frequency electrode, is provided at a predetermined distance fromthe upper side of the high frequency electrode (7). The counterelectrode (10) is electrically connected to the reaction vessel (5) tomaintain the zero potential.

The reaction vessel (5) is equipped with an etching gas inlet (11), sothat the etching gas (12) may be introduced into the reaction vessel (5)with its flow rate adjusted through the etching gas inlet (11). Thecomposition and the flow rate of the etching gas differ depending on,for instance, the reactive ion etching apparatus to be used, however, asthe etching conditions for copper or a copper alloy, there can bepreferably exemplified a total gas flow rate of 16 cc/min, comprisinggaseous NO₂ and NH₃ each flown at a rate of 3 to 9 cc/min and 13 to 7cc/min, respectively. More preferably, the etching gas is flown under acondition as such that the total gas flow rate is 16 cc/min, comprisinggaseous NO₂ and H₂ each flown at a rate of 4 to 10 cc/min and 12 to 6cc/min, respectively. For silver, gold, or an alloy containing as a maincomponent at least one of these metals, for example, the etching gas isflown under a condition as such that the total gas flow rate is 16cc/min, comprising gaseous NO₂ and NH₃ each flown at a rate of 7 to 14cc/min and 9 to 2 cc/min, respectively. More preferably, the etching gasis flown under a conditions as such that the total gas flow rate is 16cc/min, comprising gaseous NO₂ and H₂ each flown at a rate of 4 to 10cc/min and 12 to 6 cc/min, respectively.

In carrying out the etching, the reaction vessel (5) is evacuated with avacuum pump simultaneously with the introduction of an etching gas (12),such that the pressure inside of the reaction vessel (5) is maintainedin a range of 0.1 to 10 mTorr, preferably, in a range of 5 to 6 mTorr.Then, a high voltage radio frequency power is applied at a proper powerto the high frequency electrode (7) from a power supply (13) at afrequency of 13.56 MHz.

Then, the etching gas molecule introduced inside the reaction vessel (5)undergo dissociation and ionization to generate plasma. The generationof plasma concentrates at the aperture portion of the zero-potentialshield (9), and the reaction ion etching proceeds on the work pieceplaced and fixed inside the shield. The etching rate increasesapproximately in proportion to the applied radio frequency power.However, since the damage applied to the processing object increaseswith the increase in the radio frequency power, the power applied to theelectrode (7) is preferably set within the range from 50 to 150 W.

As a plasma generating apparatus, in addition to the capacitive couplingtype plasma generating apparatus above, there can be used as inductivecoupling type plasma generating apparatus, an electron cyclotronresonance type plasma generating apparatus, a helicon wave plasmagenerating apparatus, etc.

The dry etching according to the invention of the present application isdescribed in further detail below by means of examples.

EXAMPLES Example 1

A copper thin film was subjected to reactive ion etching in accordancewith process steps described below.

Referring to FIG. 1,

<a> A 1 μm thick copper thin film (2) was formed on a glass substrate(1) by means of sputtering.

<b> After forming a film of resist (3) on the product above by coating,a resist pattern was formed by electron beam lithography.

<c> Then, a fine titanium mask (4) was formed.

<d> Residual resist (3) was removed to obtain a specimen.

<e> Subsequently, the specimen thus obtained was placed and fixed on asample holder (8) provided inside a titanium reaction vessel (5) of areactive-ion etching apparatus shown in FIG. 2, and the inside of thereaction vessel (5) was evacuated while supplying gaseous NO₂ and NH₃ ata flow rate of 7 cc/min and 8 cc/min, respectively, to maintain theinner pressure at 6 mTorr. Then, high frequency power was applied at apower of 50 W to generate plasma, and reactive ion etching was performedfor 8 minutes.

<f> Thereafter, the titanium mask (4) remaining on the thus etchedcopper thin film (2) was removed by CCl₄ plasma.

The resulting product is shown in the electron micrograph given in FIG.3(a).

The portion of the copper thin film not masked by the titanium maskalone was found to be selectively etched. The selectivity ratio withrespect to the titanium mask was found to be approximately 10. Further,favorable anisotropy in etching was achieved, and the side wall wasfound to make an angle of 86° with respect to the bottom plane of thecopper thin film. Furthermore, the etching rate was found to be 55nm/min. It was confirmed that a highly efficient reactive ion etchingwas performed.

Example 2

A gold thin film was subjected to reactive ion etching in accordancewith process steps similar to those described in Example 1.

The resulting product is shown in the electron micrograph given in FIG.3(b).

The portion of the gold thin film not masked by the titanium mask alonewas found to be selectively etched. The selectivity ratio with respectto the titanium mask was found to be the same as in the case of copperthin film described in Example 1, i.e., approximately 10. Further,favorable anisotropy in etching was achieved, and the side wall wasfound to make an angle of 83° with respect to the bottom plane of thegold thin film. Furthermore, the etching rate was found to be 70 nm/min.It was confirmed that a highly efficient reactive ion etching wasperformed.

Example 3

A silver thin film was subjected to reactive ion etching in accordancewith process steps similar to those described in Example 1.

The portion of the silver thin film not masked by the titanium maskalone was found to be selectively etched. The selectivity ratio withrespect to the titanium mask was found to be approximately 12. Further,favorable anisotropy in etching was achieved, and the side plane wasfound to make an angle of 86° with respect to the bottom plane of thesilver thin film. Furthermore, the etching rate was found to be 82nm/min. It was confirmed that a highly efficient reactive ion etchingwas performed.

Example 4

A copper thin film was subjected to reactive ion etching in accordancewith process steps similar to those described in Example 1, except forusing a gaseous mixture of NO₂ and H₂ as the etching gas.

More specifically, while flowing gaseous NO₂ and H₂ at a flow rate of 12cc/min and 4 cc/min, respectively, the inside of the reaction vessel wasevacuated to maintain the pressure at 5 mTorr, and high frequency wavewas applied at a power of 50 W to perform reactive ion etching for 6minutes.

The resulting product is shown in the electron micrograph given in FIG.3(c).

The portion of the copper thin film not masked by the titanium maskalone was found to be selectively etched, and when compared it with theproduct obtained in Example 1, a sharper pattern was found to be formed.That is, this signifies that a smoother surface is obtained on thecopper thin film resulting by the etching, and that the damage to theprocessing work piece is reduced. Furthermore, the shape of the sidewalls of the copper thin film was found to be sharper and smoother. Thissignifies that the resulting product has less re-deposition layer whichthe copper thin film removed by the etching causes by again deposit tothe side walls.

The selectivity ratio with respect to the titanium mask was found to beapproximately 12. The index of anisotropy in etching, i.e., the anglebetween the bottom plane and the side plane of the copper thin film, wasfound to be 86°. Furthermore, the etching rate was found to be 120nm/min.

As a matter of course, the invention according to the presentapplication is not limited by the example described above, and it shouldbe understood that variations and modifications are acceptable on notonly the types of the etching gases, but also the details of theetching, such as the constitution and the structure of the reactive ionapparatus, as well as the operation conditions, etc.

INDUSTRIAL APPLICABILITY

As described in detail above, in accordance with the present invention,fine processing of copper, silver, gold, or an alloy containing as amain component at least one of these metals, become possible by means ofreactive-ion etching method. In addition to this, in accordance with theinvention, there is provided a mask having excellent stability and freefrom corrosion by the plasma of the etching gases wherein the etchinggases contain at least nitrogen oxide and are usable for copper, silver,gold, or an alloy containing as a main component at least one of thesemetals.

1. A dry etching method comprising etching a metallic surface of copper,silver, gold, or an alloy containing as a main component at least one ofthese metals by plasma of an etching gas containing a gaseous nitrogenoxide, hydrogen sulfide and ammonia while being reacted with the plasma.2. A dry etching method as claimed in claim 1, wherein a mask materialselected from the group consisting of titanium and a titanium alloycovers the metallic surface on etching.
 3. A dry etching methodcomprising etching a metallic surface of copper, silver, gold, or analloy containing as a main component at least one of these metals byplasma of an etching gas containing a gaseous nitrogen oxide and ammoniawhile being reacted with the plasma, wherein a mask material selectedfrom the group consisting of titanium and a titanium alloy covers themetallic surface on etching.
 4. A dry etching method as claimed in claim3, wherein the etching gas further contains hydrogen sulfide.